Tuesday, September 27, 2011

There was an interesting discussion in this month's Radiology about airport scanners (advanced imaging technology, or AIT). Two main technologies currently exist: Backscatter X-ray (ionizing radiation) and millimeter-wave (non-ionizing radiation).

Backscatter X-ray Systems

These devices transmit x-rays to passengers (depending on the manufacturer, the x-rays are either 50 kVp or 120 kVp) and detect reflected radiation. Some of the x-rays penetrate deep into the body (see below) and some bounce off the skin. The posteroanterior and anteroposterior images are obtained mainly from the reflected photons.

Given the public concern about radiation and privacy, more thought could have been put into naming these devices. Rapiscan raises the spectre of violation, the AIT84 brings to mind George Orwell's 1984, and Castscope manages to raise associations with both castration and colonoscopy.

Public fears and unfortunate names aside, the estimated radiation from these devices is pretty low. Skin dose estimates range from 0.7 μGy to 2.5 μGy and effective dose estimates from 0.015 μSv to 0.9 μSv. By way of comparison, we get an effective dose of about 0.04 μSv/minute just by flying in an airplane.

A common misconception is that photons from backscatter devices do not penetrate deep into the body (here's one from an ACR press release). While seemingly reassuring at first consideration, the implication of this misconception is that skin doses can be many times higher than the effective dose to the entire body. This misconception is the result of confusion of dose penetration with imaging penetration, the former reflecting the behavior of the photons in the body and the latter describing the photons used for image creation (more here).

The penetration of the photons into the body does raise concern about fetal dose. However, a scan delivering an effective dose of 0.25 μSv to the mother is estimated to deliver only about 0.12 μSv to the uterus.

Millimeter-Wave Systems

Another system used in airports makes use of non-ionizing, millimeter-wave radiation. These come in active and passive varieties. Active scanners direct millimeter-wave energy at the passenger, while passive systems detect energy naturally emitted from the body and concealed objects. The data is then analyzed and reconstructed to generate a 3-dimensional holographic image (as opposed to the planar images generated by the backscatter devices).

L3 Security and Detection Systems manufactures the ProVision system currently used by the TSA.

References

Brenner DJ. Are x-ray backscatter scanners safe for airport passenger screening? For most individuals, probably yes, but a billion scans per year raises long-term public health concerns. Radiology. 2011 Apr;259(1):6-10.